The present invention relates to a medical device and more particularly to a stent for use with an expandable device for repairing diseased blood vessels.
The vasculature of an animal or a human characteristically suffers from a variety of maladies. Vessel walls can weaken and become distended over time in response to blood flow and pressures, thereby resulting in formation of aneurysms. Such aneurysms can take on a myriad of forms. In particular, aneurysms may form at or near bifurcated vessels creating enlarged areas about the bifurcation, or may form a pocket, for example, in side walls of vessels. Due to the complications associated with aneurysms that rupture or otherwise fail, it is critical that an aneurysm be treated expeditiously and effectively. Intravascular treatment procedures include placing grafts within the aneurysm in a manner to ensure that blood flows through the graft rather than through the weakened vessels. Grafts are sometimes anchored at the repair site using expandable stents, precise placement of which is often highly desirable.
Moreover, aneurysms in the form of pockets in the sidewall of a vessel may be repaired using stents. In the event such an aneurysm is formed in a patient""s vasculature, the pocket defined by the aneurysm might first be segregated from the parent vessel by placing a stent across the aneurysm neck, and then filling the aneurysm with embolic material. The stent limits flow of blood into the aneurysm and prevents embolic material from herniating into the parent vessel. Again, precise placement of the stent is often highly desired.
Stenoses also typically form in vasculature of humans and animals. Specifically, thrombotic or atherotic stenoses can form nearly anywhere in the vasculature. Such narrowing of the vessel is, of course, highly dangerous to the patient where the afflicted vessel provides the sole blood flow access to critical parts of the body. To treat such stenoses, a supporting structure can be placed at the diseased site for the purpose of enlarging and holding open the vessel. It is known in the art to employ stents for this purpose.
Various means have been provided to deliver and implant stents for the purpose of facilitating the repair of narrowed or weakened vessels. One method frequently described for delivering a stent to a desired intraluminal location includes mounting the expandable stent on an expandable member, such as a balloon, provided on the distal end of an intravascular catheter. The assembly is then advanced to the desired location within the patient""s body lumen, the balloon on the catheter is expanded to open the stent into a permanent expanded condition and then the balloon is deflated and the catheter removed from the vasculature.
It has become desirable to perform such procedures in the cerebral vasculature to open vessels that are narrowed by fibrins, platelets, plaque or calcium or to reinforce weakened vessels such as those suffering from aneurysms. Due to the threat of stroke and the need to maintain bloodflow to the brain, it is particularly desirable to treat such vessel abnormalities as expediently and effectively as possible. Since cerebral vasculature is characterized by having numerous perforators branching from main pathways and since such perforators often provide the only blood source to a particular part of the brain, accurate placement of the stent within cerebral vasculature is highly critical. Unfortunately, stents often move longitudinally within the vessel during expansion, often resulting in inaccurate placement.
Furthermore, many conventional stents shorten when they expand from a contracted state to an expanded state. Such shortening often occurs due to forces applied by the catheter used in deployment. In particular, the balloon portion of a balloon catheter, upon expansion, can generate inwardly directed loads upon the ends of the stent thereby causing the stent to shorten in length. This shortening is typically variable and difficult to quantify reliably which adversely effects the accurate placement of the stent within vasculature.
Additionally, expansion of the stent can itself be traumatic to a vessel wall. That is, uncontrolled expansion can result in the stent forcefully engaging and thereby damaging a vessel wall. Thus, a high rate of expansion of the stent may be undesirable.
Accordingly, what is needed is a stent that can be accurately placed within vasculature and that can be repositioned at some intermediate stage of expansion that is less than the final expanded size but greater than the original reduced size. It is also desirable that the stent embodies structure that offsets the shortening of the stent due to balloon expansion forces. Controlled expansion of the stent is also desirable. The present invention satisfies these needs.
Briefly, and in general terms, the present invention provides a medical device that includes structure that is adapted to minimize the effect of opposing forces typically applied by expandable members of a delivery catheter. The medical device of the present invention also includes structure that facilitates staged expansion of the stent.
In a preferred embodiment, the medical device embodies a stent that has a generally overall tubular configuration. The stent body is defined by a pattern of repeating and symmetrical beam elements that share a common longitudinal axis. In a preferred configuration, the repeating beam elements form an annular element embodying a circumferentially directed wave form. The beam elements can also form a helical pattern. The beam elements are characterized by including low force actuated beams and high force actuated beams. Through variance of dimensions of the two types of beam elements, full expansion of the low force elements can be achieved prior to expansion of the high force elements, resulting in a staged expansion characteristic of the stent.
The present invention also provides a stent which expands in a more controlled fashion. Specifically, the contemplated stage expansion reduces the rate of expansion of the stent of the present invention, thereby facilitating the minimization of the forces applied to the wall of a target vessel characteristic to uncontrolled expansion. Additionally, the staged expansion facilitates repositioning of the device at an intermediate stage of expansion.
It is to be recognized that as the low force actuated beams are deflected, the force required to continue deflection increases. Eventually, the force required is great enough to begin deflecting the large force actuated beams. At this point, a second phase of expansion begins. By varying the number and quality of the high and low force actuating beams, a desired stage expansion and rate of expansion is achieved.
These and other objects and advantages of the invention will become apparent from the following more detailed description, when taken in conjunction with the accompanying drawings of the illustrated embodiments.